The present invention relates to self-corrected electric filters and more specifically to self-corrected electric filters with localized constant elements of the band pass type making it possible to filter analog or digital signals in a wide frequency band and making it possible to correct the group or envelope delay time distortion in this frequency band.
Such filters are particularly useful in radio receivers. In a particular construction of such receivers, conventional electric filters are used, which do not correct the distortion of the group delay. However, this is often an important disadvantage for the transmission of the signal. The passage time at the different frequencies of the pass band is not constant. Thus, the group delay increases from a low value for the centre frequencies of the pass band up to a high value for its limit frequencies. The group delay correction function is realised by two correcting networks which are very frequently formed by one or more cells comprising active elements. When there are several cells in the correcting network, each cell is insulated from the adjacent cells by a transistor. These correcting networks are arranged in cascade form following the filter. In the case of a radio receiver, it is conventional practice to use two cells in cascade for phase correction purposes. The centre frequencies of the two cells are located on either side of the centre frequency of the band pass filter which it is sought to compensate.
In certain ultra-high frequency applications, narrow pass band microwave filters are used. The group delay correction function is realised by using multiple-coupled cavities. The synthesis of these filters have been described in the technical literature, as is shown by the following publications: J. D. RHODES "General filter with linear phase by direct coupling cavity" i.e. IEEE Transactions MTT, Volume MTT 18, No. 6, June 1970, pp. 308-313; A. E. Atia et al "Narrow-band pass waveguide filters", IEEE Transactions MTT, Volume MTT 18, No. 4 April 1972, pp. 258-264; and A. E. Atia et al "Synthesis of narrow-band multiple-coupled cavity", IEEE Transactions CAS, Volume CAS-21, No. 5, September 1974, pp. 649-655.
The disadvantages of these two filter types will be described hereinafter. Multiple-coupled cavity and narrow-band pass waveguide filters cannot be used at low or intermediate frequencies due to the rating and excessive overall dimensions. Conventional filters equipped with correcting networks require the use of special supply systems for active cells. In addition, they have large overall dimensions and are not always reliable.